1. Field of the Invention
The present invention relates to a bi-directional DC/DC converter of an isolation type and resonance type applied to devices with a wide range of input and output voltage, such as a battery charger.
2. Description of the Related Art
FIG. 4 shows a traditional bi-directional DC/DC converter of an isolation type. The bi-directional DC/DC converter comprises: a first DC voltage source 1 with a DC voltage of V1, a second DC voltage source with a DC voltage of V2, smoothing capacitors 3 and 4, an isolation transformer 17, a first bridge circuit 18, a second bridge circuit 19, and a smoothing reactor 24. The transformer 17 has a primary winding N1 and a secondary winding N2, the numbers of winding thereof being represented by the same symbols N1 and N2.
The first bridge circuit 18 has semiconductor switching elements and converts a DC power to an AC power when the power flow is from the first DC voltage source 1 to the second DC voltage source 2 and converts an AC power to a DC power when the power flow is from the second DC voltage source 2 to the first DC voltage source 1. The second bridge circuit 19 has semiconductor switching elements and converts a AC power to an DC power when the power flow is from the first DC voltage source 1 to the second DC voltage source 2 and converts a DC power to an AC power when the power flow is from the second DC voltage source 2 to the first DC voltage source 1.
The semiconductor switching elements composing the first and second bridge circuits 18 and 19 can be a reverse-conducting element such as an IGBT or a MOSFET having an antiparallel-connected diode.
Patent Document 1 (identified further on) discloses a conventional technology equivalent to a bi-directional DC/DC converter of this type.
Each of the first and second bridge circuits 18 and 19 in FIG. 4 exhibits both the capabilities of converting an AC power to a DC power and converting a DC power to an AC power. Consequently, the bi-directional DC/DC converter of FIG. 4 does not need to be equipped with distinct dedicated circuits corresponding to the power flow direction and can be a simplified, small-scale device.
The bi-directional DC/DC converter disclosed in Patent Document 1 conducts hard switching based on pulse width modulation (PWM) control. As a result, when the power flow is from the first DC voltage source 1 to the second DC voltage source 2, the semiconductor switching element is subjected to a surge voltage V2+ΔV exceeding the voltage V2 of the second DC voltage source 2 across the switching element during the time the semiconductor switching element of the second bridge circuit 19 in rectifying operation is turning OFF in which the diode is in a reverse recovery process. As a result, semiconductor switching elements composing the second bridge circuit 19 are necessarily semiconductor switching elements with high withstand voltage, which generally produce large loss. This causes a problem of low efficiency of the device. This problem arises as well in the first bridge circuit 18 in the case of the power flow from the second DC voltage source 2 to the first DC voltage source 1.
A conventional technology to solve this problem is disclosed in Patent Document 2 (identified further on), for example, which is a resonance type bi-directional DC/DC converter performing pulse frequency modulation (PFM) control utilizing a resonance phenomenon of an LC resonance circuit.
FIG. 5 shows an example of construction of the main circuit of the conventional resonance type bi-directional DC/DC converter. The circuit components in FIG. 5 serving the same function as those in FIG. 4 are given the same symbols and the description thereof is omitted, and the different points are mainly explained here.
Referring to FIG. 5, this resonance type bi-directional DC/DC converter comprises resonance reactors 13 and 14, and resonance capacitors 15 and 16. The first bridge circuit 18 is composed of IGBTs 5 through 8, which are semiconductor switching elements, each having an anti-parallel-connected diode, and the second bridge circuit 19 is composed of IGBTs 9 through 12. The gate terminals of the IGBTs 5 through 12, as well as the gate signals, are designated by the symbols G1 through G8.
FIGS. 6 and 7 show construction of control means for generating the gate signals G1 through G8 for the IGBTs 5 through 12.
FIG. 6 shows a construction for generating the gate signals G1 through G4 for the IGBTs 5 through 8, which comprises a second detection circuit 21 for detecting the voltage V2 and the current I2 of the second DC voltage source 2, and a first control circuit 25 for generating the gate signals G1 through G4 according to the detected values by the detection circuit 21. FIG. 7 shows a construction for generating the gate signals G5 through G8 for the IGBTs 9 through 12, which comprises a first detection circuit 20 for detecting the voltage V1 and the current I1 of the second DC voltage source 1, and a second control circuit 26 for generating the gate signals G5 through G8 according to the detected values by the detection circuit 20. All the gate signals G1 through G8 are given through a respective gate driving circuit (not shown in the figure) to the IGBTs 5 through 12.
In the case the power flow is from the first DC voltage source 1 to the second DC voltage source 2 in the DC/DC converter of FIG. 5, the voltage of each of the semiconductor switching elements 9 through 12 in rectifying operation is clamped at the voltage V2 of the second DC voltage source 2 in the process of reverse recovery of the accompanying diodes. In the case the power flow is from the second DC voltage source 2 to the first DC voltage source 1, the voltage of each of the semiconductor switching elements 5 to 8 in rectifying operation is clamped at the voltage V1 of the second DC voltage source 1.
Consequently, the semiconductor switching elements 5 through 12 used here can be semiconductor switching elements with a low withstand voltage that generally produce a small loss. Thus, the bi-directional DC/DC converter of FIG. 5 exhibits higher efficiency than the bi-directional DC/DC converter of FIG. 4.
As described above, a resonance type bi-directional DC/DC converter that performs frequency modulation control can improve efficiency of the converter.
However, as pointed out in Patent Document 3 (identified further on), a characteristic of output voltage versus switching frequency changes depending on a magnitude of the load. In a case of light load or no load, in particular, the output voltage cannot be decreased below a certain value even if the switching frequency is infinitely increased. Therefore, it can be hard to apply the bi-directional DC/DC converter of FIG. 5 to devices with a wide range of input/output voltage, such as battery chargers.    [Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2001-037226 (paragraphs 0016-0041, FIG. 2, in particular)    [Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2011-120370 (paragraphs 0010 through 0044, and FIGS. 1 and 2, in particular)    [Patent Document 3]
Japanese Unexamined Patent Application Publication No. 2002-262569 (paragraphs 0002 and 0003, in particular)
The problem pointed out in Patent Document 3 is mentioned specifically in the following.
The resonance type bi-directional DC/DC converter of FIG. 5 is assumed to specify that the voltage of the first DC voltage source 1 is V1 and the voltage of the second DC voltage source 2 is controlled in the range from V2min to V2max. In the design of the converter based on the operation with the power flow from the second DC voltage source 1 to the second DC voltage source 2, a bi-directional DC/DC converter of a resonance type cannot deliver an output voltage below a certain value in a light load or no load condition. Thus, the winding ratio “a” of the isolation transformer 17 is a=N1/N2=V1/V2min.
Consequently, the minimum output voltage V2min from the second bridge circuit 19 in the case of the power flow from the first DC voltage source 1 to the second DC voltage source 2, is V2min=(1/a)×V1. Here, the switching frequency of the IGBTs 5 through 8 is set at the resonance frequency fr of the LC circuit composed of the resonance reactor 13 and the resonance capacitor 15. The maximum output voltage V2max in this case is V2max=(1/a)×V1×α, in which α is a voltage gain when the switching frequency is set at a value lower than the resonance frequency fr.
Thus, a voltage in the range from V2min to V2max is delivered.
Next, the case of power flow from the second DC voltage source 2 to the first DC voltage source 1 is considered. When the input voltage to the second bridge circuit 19 is the minimum input voltage V2min, the voltage V1=a×V2min. Thus, the voltage V1 can be delivered by setting the switching frequency of the IGBT 9 through 12 at the resonance frequency fr. When the input voltage to the second bridge circuit 19 is the maximum input voltage V2max, however, V1<a×V2max. Thus, the switching frequency needs to be made higher than the resonance frequency fr.
Since the output voltage of a bi-directional DC/DC converter of a resonance type cannot be controlled below a certain value even if the switching frequency is increased infinitely, a desired output voltage V1 would not be obtained in a light load or no load condition.
Therefore, it can be hard to apply a resonance type bi-directional DC/DC converter using pulse frequency modulation control to devices with a wide range of input/output voltage.